LeetCode #219 Contains Duplicate II Solution & Explanation

public class Solution {
    public bool ContainsNearbyDuplicate(int[] nums, int k) {
        Dictionary<int,int> dict =   new Dictionary<int,int>();

        for(int i = 0; i<nums.Length; i++)
        {
            if (!dict.ContainsKey(nums[i]))
            {
                dict.Add(nums[i], i);
            }
            else
            {
                if(i-dict[nums[i]]<=k)
                {
                    return true;
                }
                dict[nums[i]] = i;
            }
        }
        return false;
    }
}

class Solution {
    public boolean containsNearbyDuplicate(int[] nums, int k) {
        Map<Integer,Integer>map =   new HashMap<Integer,Integer>();

        for(int i = 0; i<nums.length; i++)
        {
            if (map.containsKey(nums[i]))
            {
                if(i-map.get(nums[i])<=k)
                {
                    return true;
                }   
            }
            map.put(nums[i],i);
        }
        return false;
    }
}

class Solution:
    def containsNearbyDuplicate(self, nums: List[int], k: int) -> bool:
        dict = {}

        for i in range (len(nums)):
            if (nums[i] in dict):
                if(i-dict[nums[i]]<=k):
                    return True

            dict[nums[i]] = i
        return False

/**
 * @param {number[]} nums
 * @param {number} k
 * @return {boolean}
 */
var containsNearbyDuplicate = function(nums, k) {
        const map =   new Map();

        for(let i = 0; i<nums.length; i++)
        {
            if (map.has(nums[i]))
            {
                if(i-map.get(nums[i])<=k)
                {
                    return true;
                }   
            }
            map.set(nums[i],i);
        }
        return false;
};